Spanning Tree Protocol (STP), which is applied to a Layer 2 (L2) network coupling servers in a data center, is a technique for preventing a loop from being formed in the L2 network by setting a blocking port in a relay device in the network. However, part of use of a path in the network is limited when the blocking port is set. Thus, a network band is not efficiently used.
As the traffic in a data center network has increased in recent years, international standardization of a multipath technique that is applied to the L2 network has progressed so that the network band is efficiently used. TRansParent Interconnection of Lots of Links (TRILL), which is one of the multipath techniques, has been proposed to prevent a loop from being formed in the L2 network coupling the servers.
FIG. 1 illustrates an example of an information processing system to which the TRILL is applied. The example of the information processing system includes a network that has six routing bridges and four servers coupled to the network. In FIG. 1, an RB indicates a routing bridge, and an SV indicates a server. Among the six routing bridges, a routing bridge which is coupled to an external device (for example, a server SV) of the network may be referred to as an edge routing bridge. The coupling between the routing bridges is referred to as a link. A parameter called a link cost is set to the link. The coupling between the edge routing bridges is referred to as a path. The total value of the link cost that is set to the link included in the path is defined as the total link cost. When the six routing bridges are coupled to each other, a plurality of paths is formed. The path of which the total link cost is the smallest is selected if there is more than one path that is selectable to be used for transfer between the servers. If there is more than one path having the smallest total link cost, a load of the network is dispersed when an algorithm for selecting the paths at the same ratio is applied.
There is known a technique of a relay device in the network for collecting flows of an IP datagram and distributing transfer destinations by the collected unit.    [Patent Document] Japanese Laid-open Patent Publication No. 2000-253058